Chiral nematic (also referred to as cholesteric) liquid crystals can be made to exhibit different optical states under different electrical field conditions and are characterized by a unique combination of properties, including optical bistability, making them particularly useful in display applications. Depending upon the magnitude and shape of the electric field, the optical state of the material can be changed between light scattering and light reflecting states, or any one of a number of intermediate states there between which can be made to reflect any desired intensity of colored light along a continuum of such states, thus providing gray scale.
With such chiral nematic materials, a low electric field pulse generally results in a light scattering focal conic texture. The application of a sufficiently high electric field pulse, i.e., an electric field high enough to homeotropically align the liquid crystal directors, will, upon removal of the pulse, drive the material to a light reflecting twisted planar texture that can be any desired color depending upon the pitch length of the liquid crystal. The light scattering and light reflecting states can be made to remain stable at zero field. If a sufficiently high electric field is maintained, the material is transparent until the field is removed. When the field is turned off quickly, the material reforms to the light reflecting state and, when the field is turned off slowly, the material reforms to the light scattering state. Electric field pulses of various magnitudes below that necessary to drive the material from the stable reflecting state to the stable scattering state will drive the material to intermediate states that are themselves typically stable. These multiple stable states indefinitely reflect colored light of an intensity between that reflected by the reflecting and scattering states. Thus, depending upon the magnitude of the electric field pulse the material exhibits stable gray scale reflectivity. Application of mechanical stress to the material can also be used to drive the material from the light scattering to the light reflecting state.
The color reflected by a chiral nematic liquid crystal depends upon the pitch length of the liquid crystal, which is in turn dependent upon the amount of chiral material in the liquid crystal. The pitch length of the liquid crystal materials may be adjusted based upon the following equation (1):λmax=navpo  (1)where λmax is the peak reflection wavelength, that is, the wavelength at which reflectance is a maximum, nav is the average index of refraction of the liquid crystal material, and po is the natural pitch length of the chiral nematic helix. Definitions of chiral nematic helix and pitch length and methods of its measurement, are known to those skilled in the art such as can be found in the book, Blinov, L. M., Electro-optical and Magneto-Optical Properties of Liquid Crystals, John Wiley & Sons Ltd. 1983.
The wavelength of the reflected light can also be controlled by adjusting the chemical composition, since cholesterics can either consist of exclusively chiral molecules or of nematic molecules with a chiral dopant dispersed throughout. In this case, the dopant concentration is used to adjust the chirality and thus the pitch. For most concentrations of chiral dopants, the pitch length induced by the dopant is inversely proportional to the concentration of the dopant. The proportionality constant is given by the following equation (2):po=1/(HTP.c)  (2)
where c is the concentration of the chiral dopant and HTP (helical twisting power) is the proportionality constant.
One of the original approaches to photo-tuning of helical pitch length and λmax, of cholesteric liquid crystals was to add chiral dopants that were photoactive (see, A. Yu. Bobrovsky, N. I. Boiko and V. P. Shibaev Liq. Cryst. 25 (1998), p. 679.; A. Yu. Bobrovsky, N. I. Boiko and V. P. Shibaev Liq. Cryst. 26 (1999), p. 1749; P. van de Witte, J. C. Galan and J. Lub Liq. Cryst. 24 (1998), p. 819; M. Brehmer, L. Lub and P. van de Witte Adv. Mater. 10 (1998), p. 1438).
The main principle of the development of such light-controllable liquid crystal is based on the synthesis of photochromic copolymers whose macromolecules consist of mesogenic (as a rule, nematogenic) and combined photo-tunable chiral dopant (PTCD) groups, which are chemically linked in the common monomer unit. In this case, mesogenic fragments are responsible for the formation of the nematic phase, chiral groups provide the twisting of the nematic phase and formation of helical supramolecular structure. Finally, photo-tunable chiral dopant fragments can easily change their molecular structure under the light irradiation.
The irradiation with a certain wavelength leads to the photo-induced transformations of the photo-tunable chiral dopant affecting both the configuration and shape of the side-chain group. This leads to a decrease both in the anisotropy of photo-tunable chiral dopant group and the helical twisting power of a given chiral group. A decrease in helical twisting power leads to the untwisting of the cholesteric helix, which is accompanied by a shift in the selective light reflection maximum to longer wavelengths. Thus, using light irradiation as the external control factor, one may effectively modify the optical properties of polymer films by changing the local supramolecular helical structure.
As noted above, selective adjustment of the pitch length of a liquid crystal, and hence the color reflected thereby, can be accomplished by using photo-tunable chiral dopants (PTCDs). Irradiation of a photo-tunable chiral dopant with, for example, ultra violet (UV) light or other high energy source such as laser, results in conversion of chiral photo-tunable chiral dopant to an achiral molecule or to a racemic mixture. When one or more photo-tunable chiral dopants are included in a chiral nematic liquid crystal material, the pitch length of the resulting liquid crystal mixture can be either extended or shortened by varying exposures to UV light. By irradiating different regions of the material with different amounts of UV through the use of masking techniques, the pitch lengths of each region can be tuned to reflect a different color, thereby creating different colored pixels or regions of spot color in the liquid crystal material itself.
U.K. Patent Application No. GB 2355720 discloses a process of preparing a reflective film by using a photodegradable chiral compound. GB 2355720 describes a process for preparing a reflective film by coating a polymerizable cholesteric liquid crystal (CLC) material onto a substrate, aligning the material into planar orientation, and polymerizing the material by exposure to actinic radiation, characterized in that the polymerizable material comprises at least one photodegradable chiral compound that loses its chirality when being exposed to actinic radiation. Also disclosed is the use of said reflective film in optical, electrooptical, information storage, decorative and security applications, a liquid crystal display device, and a photodegradable compound.
U.S. Pat. No. 5,668,614 discloses a tunable chiral material (TCM) that can be changed from chiral to achiral or to a racemic mixture by irradiating with, for example, UV light or a high energy source such as laser. Further disclosed is a light modulating liquid crystal cell comprising a light modulating chiral nematic liquid crystal material including a tunable chiral material, wherein different regions of the liquid crystal material exhibit different reflection wavelengths. The cell is prepared by partially exposing the liquid crystal material with the tunable chiral material to photo-irradiation, e.g. through a photomask, leading to a change of the chirality of the tunable chiral material and thus to a change of the helical pitch in the exposed parts of the chiral nematic liquid crystal material.
WO 98/57223 discloses a multi domain liquid crystal display device comprising a layer of nematically ordered liquid crystalline material containing a chiral dopant sandwiched between two substrates. The liquid crystal layer comprises at least two types of sub-pixels in which the twist senses of the liquid crystalline material are mutually opposite, and the composition of the chiral dopant in the different types of sub-pixels is different. The device is manufactured by sandwiching between the substrates a layer of liquid crystalline material containing an isomerisable chiral dopant with a first twist sense and a non-isomerisable chiral dopant with an opposite twist sense, and photoirradiating the layer through a photomask. This causes the isomerisable dopant in the exposed parts of the layer to convert its chirality and thus its twist sense, leading to a change of the helical pitch in the exposed parts.
However, in order to achieve desired change in chiral materials as described in U.S. Pat. No. 5,668,614 and WO 98/57223, irradiation with UV light of high intensity and long duration is required. Therefore high lamp powers and long irradiation times are needed, which is a serious drawback for mass production. This is especially disadvantageous in case the isomerizable chiral compound is used in a photocurable or photopolymerizable liquid crystal mixture, where the light used to induce a change in the isomerizable chiral compound also has the undesirable effect of inducing a premature polymerization process in the mixture. Furthermore, UV irradiation of the mixture with high radiation doses (i.e. high radiation intensities and long radiation periods) can cause undesired degradation of the other components of the liquid crystal mixture.